Every time after my Mac (MBP late 2013 with OS X 10.10.5) starts Screen Saver mode and then after some time turns off the screen (even if no program is running) the fan speed rises dramatically.
Is it normal, or does anyone know how to fix this?

Are you using one of the native apple's screen savers or have you installed a fancy one, downloaded from the internet? I ask this because, for instance, a matrix-like screensaver (with 100+ falling green signs) can be really cpu demanding. This can cause the fans to be started...
– Pierre WateletAug 21 '15 at 14:25